"The Infinite Hotel, a thought experiment created by German mathematician David Hilbert, is a hotel with an infinite number of rooms. Easy to comprehend, right? Wrong. What if it's completely booked but one person wants to check in? What about 40? Or an infinitely full bus of people?" A fun thought experiment to visualize the concept of infinity. Your brain starts to hurt at the two-and-a-half-minute mark. The full TED-Ed lesson is here.